The body of the Stephen Paddock, who sprayed more than 1,000 bullets into a Las Vegas country music concert, is to be sent to Stanford University for study.

Las Vegas coroner John Fudenberg said an autopsy was completed on the 64-year-old, but a finding on a cause and manner of his death is not expected for several months.

Investigators have also given more details on the chronology of events surrounding the shooting and pushed back against criticism that they were changing their story. Shifting accounts about when Paddock fired his first shots in his 32nd floor Mandalay Bay suite have led to questions about whether police could have done more to stop him on October 1.

"In the public space, the word 'incompetent' has been brought forward," Clark County Sheriff Joe Lombardo said. "I am absolutely offended with that characterisation."

In a chronology provided on Monday, Mr Lombardo had said Paddock started spraying 200 rounds from his suite into the hallway of the Mandalay Bay at 9.59pm, wounding an unarmed security guard in the leg. He said on Friday that the security guard came to a barricaded stairwell door at 9.59pm and was not shot until around 10.05pm.

About that time, the gunman unleashed a barrage of bullets on the festival crowd, killing 58 people. Then he killed himself with a gunshot to the head.

The timeline comes as investigators say they have yet to identify a motive behind the worst mass shooting in modern US history. The FBI said agents have conducted hundreds of interviews, chased nearly 2,000 leads, looked at Paddock's computers and phone, collected 1,000 pieces of evidence, and analysed hours of video footage.

"We are establishing a timeline of this suspect's life, his motivation and everybody associated with him throughout time," Mr Lombardo said.

The sheriff became emotional describing gunshot wounds one on-duty officer, Brady Cook, received to the shoulder, bicep, chest and back as he arrived in a police patrol car moments after shooting started.

"It is readily apparent to me that (Paddock) adjusted his fire and directed it toward the police vehicles," Mr Lombardo said.

"No matter what his personal vendetta is against the police or not, maybe he was preventing the wolf from getting to his door sooner than later, but he chose to fire upon police vehicles."

A visual inspection of Paddock's brain during the coroner's autopsy found "no abnormalities," Mr Lombardo said.

Mr Fudenberg said he would await findings of multiple forensic analyses at Stanford, including a neuropathological examination of Paddock's brain tissue, before issuing a finding on a cause and manner of his death.

The sheriff also said the FBI is now taking on a greater role in the investigation.

Mr Lombardo's newest version of events aligns with what Mandalay Bay owner MGM Resorts International said on Thursday. They had disputed whether six minutes actually passed between the first shots in the hallway and the start of the concert rampage and said Paddock may have wounded the security guard within 40 seconds of firing into the crowd.

Earlier this week, lawyers had questioned why police and security were not able to stop Paddock sooner when Mr Lombardo said six minutes passed between the shooting of guard Jesus Campos and the gunfire into a crowd of 22,000 at the Route 91 Harvest Festival concert.

The 10-minute attack on the crowd began at 10.05pm, when the 64-year-old real estate investor, high-stakes video poker player and retired accountant began firing more than 1,000 rounds from two bashed-out windows, police said. Officers arrived on the 32nd floor at 10.17pm, two minutes after he had stopped shooting, Mr Lombardo said.

The wounded Mr Campos used his radio and mobile phone to call for help, police said. A maintenance worker, Stephen Schuck, has said he also called for help on his radio, asking a dispatcher to call police because someone was shooting a rifle on the 32nd floor.

It is not clear what Mandalay Bay maintenance and security workers did with those radio messages.

The timeline given by police earlier this week differed dramatically from the one they gave last week: that Paddock wounded Mr Campos after he had fired on the crowd.

Mr Lombardo confirmed that Paddock intentionally opened fire on jet fuel tanks at the nearby McCarran International Airport and said he took shots at arriving police officers, possibly to keep them at bay as police rushed to his room.